Monitoring pipes

ABSTRACT

An apparatus as disclosed for mounting a liquid-filled pipe by the use of ultrasound techniques. The apparatus comprises a vehicle capable of fitting inside the pipe, and of being transported along the pipe by the liquid flowing into it, together with rings of several ultrasonic transducers spaced circumferentially round the vehicle. The rings are so arranged that one ring operates in the reflection mode purely radially while two further rings operate as a pair in a longitudinal refraction mode. A fourth ring is used in a circumferential refraction/reflection mode. The vehicle has an associated power supply, data gathering and storage capability and a vehicle for measuring the distance traveled along the pipe.

This invention relates to monitoring pipes, and refers in particular tothe use of ultrasonics to characterize liquid-filled pipes.

The owners of liquid-filled pipelines frequently wish to monitor thecondition of those pipes. The uncertainties may include the materialtype, internal diameter, and thickness of the pipe, the material type,and thickness of any internal lining if present, the presence ofcorrosion on the internal surface of the pipe, the presence of corrosionon the external surface of the pipe, damage to the lining, the thicknessof any deposits of material on the internal surface of the pipe or ofthe lining if present, the presence and extent of circumferentialcracks, the presence and extent of longitudinal cracks, and the positionof features such as bends, offtakes, valves and joints. In view of thehigh cost of replacing pipework, and the potential consequential damagedue to fluid loss from a break in the pipework, it can be justifiablefor the owner to carry out regular surveys of the condition of thepipework from within the pipe using a vehicle transported along insidethe pipe. In some circumstances it is be important not to disturb anydeposits on the pipe wall. In these cases the condition needs to beassessed without contacting the wall.

So-called ‘intelligent pigs’ are frequently used inside high-pressuregas pipelines to detect the presence of corrosion and other defects. Theearliest vehicles were magnetic flux leakage devices. A powerfulmagnetic field is used to saturate the steel wall, and any defects inthe pipe wall induce anomalies in the magnetic field downstream whichare subsequently detected by the ‘pig’. This device requires steel pipewith no internal lining, with no internal deposit, demands close contactwith the wall, and has a high power requirement. Thus, it fails to matchmany of the requirements described above. Even in steel pipes the fluxleakage method fails to detect longitudinal cracks very well. Themagnetic flux leakage method has been applied successfully tofluid-filled unlined steel pipes, but the success was contingent uponthe pipes being carefully cleaned of any deposit before the flux leakagedevice was used.

To detect longitudinal cracks, other vehicles inside high-pressure gaspipelines use ultrasonic transducers in shear mode. Fluid-filled resinwheels are pressed closely against the wall of the pipe, and used tocouple ultrasonic shear waves into the wall. Ultrasonic waves in thisinstance refer to elastic waves at high frequencies in the hundreds ofkilohertz (and even low megahertz) ranges. A compression wave isanalogous to the visible motion transmitted along a stationary line ofrailway wagons when struck by a heavy diesel locomotive. A shear wave isanalogous to the manner in which a side-winder snake manages to makeforward progress by wriggling its body. The shear waves induced by thefluid-filled resin wheels travel circumferentially around the pipe, andare detected by adjacent wheels. In principle this technique can detectlongitudinal cracks. It is evident though, that in many respects themethod fails to address the full problem of condition monitoringdescribed above. The fluid-filled wheels are required inside the gaspipe because the high impedance mismatch between gas and solid result ina near total reflection at the inner surface of the pipe if the waveswere launched in the gas. This limitation does not apply inliquid-filled pipes for compressive waves. The liquid can provide a goodcoupling between compressive waves in the liquid and in the solid.

In the oil industry, ultrasonics have been used in reflection mode ascaliper tools to measure the diameter of wells deep underground. Suchwells are generally filled with liquids of various types. A toolinserted into the well projects an ultrasonic compression pulse radiallythrough the liquid, and measures the time take for the first reflectionto return from the wall. After the well has been drilled it is usuallylined with a steel casing to keep it open. Cement is injected behind thecasing into the space between the casing and the rock as drilled. Anultrasonic tool is sometimes used in cased wells to check the bondbetween the casing and the cement. An ultrasonic pulse penetratesthrough the steel casing, through the cement, and into the rock. Thequality of reflection from the casing-to-cement interface is a measureof the quality of the cement bond to the casing. A good bond gives alower-magnitude reflected pulse than the situation where a fluid-filledmicro-annulus has developed between the casing and the cement. Anotheroil industry tool uses ultrasonic waves in refraction mode. Anultrasonic pulse is projected at an angle to the steel wall, and itcouples by refraction into the casing and into the rock behind thecasing. The transmitted pulse is detected by a receiver also at an angleto the wall, so a second refraction is required to detect thetransmitted signal. The presence of a micro-annulus indicating a poorcement bond prevents good transmission into the rock, and this gives areceived signal which differs from that which obtains when the bond is agood one.

Another method used inside steel pipes is commonly referred to asremote-field eddy current. An induction coil creates a magnetic fieldaxially along the pipe whose return path is partly along the pipe walland partly along the surrounding medium. The flux normal to the pipewall is measured by probes close to the wall. Defects cause measurabledisturbances in that flux. The method cannot be used in non-magneticpipe, and gives no information about lining or deposits. As withmagnetic flux-leakage devices, this device requires the wall innersurface to be cleaned of any deposits before being used.

Ultrasonic methods are used inside pipes in a reflection mode todetermine pipe diameters and to detect the presence of a micro-annulusin cement surrounding the pipewall. They are also used in refractionmode to detect the presence of a micro-annulus in cement surrounding thepipewall. They have also been used in both reflection and refractionmodes to detect flaws in pipes. However, in all these cases the materialof the wall is assumed to be known so that the speed of sound in thewall material is known. And in none of the above cases is there presumedto be any lining or deposit on the inner surface of the wall. Thisinvention sets out to combine ultrasonic methods in reflection,refraction and reflection-refraction modes so as to identify thematerials of the pipewall, the lining and any interior debris prior tousing this information to size the pipewall, radius and thickness, thelining and any debris and then to identify features such as cracks,corrosion and fittings.

The invention proposes for this purpose Apparatus for thecharacterization of a liquid—filled pipe, which Apparatus comprises:

1. a vehicle capable of fitting inside the pipe and of being transportedby the liquid along the pipe.

2. the vehicle carrying rings of several ultrasonic transducers,preferably disposed at equidistance circumferentially around the ringand the rings arranged such that:

2.1 one ring of transducers operates in the reflection mode purelyradially.

2.2 two rings of transducers operate as a pair displaced axially to eachother. The pair is used in a longitudinal refraction mode. One ring oftransducers emits pulses along the pipe at the critical angle to thewall such that the wave is refracted as waves travelling within thedebris, lining and pipewall, and the refracted wave is received by theother ring of transducers.

2.3 the emit ring of transducers of the refraction mode pair also hasthe capability of receiving reflected echoes, this being arefraction/reflection mode.

2.4 one ring of transducers is used in a circumferentialrefraction/reflection mode. Each transducer emits pulses in a radialplane but at the critical angle circumferentially to the wall such thatthe wave is refracted as a circumferential wave along the debris, liningand pipewall and the transducer receives reflections back along the samepath.

3. the vehicle carrying power to supply the ultrasonics and associateddata gathering and data storage.

4. the vehicle carrying data storage capacity to save the ultrasonicinformation which is captured at regular intervals of distance along thepipe.

5. the vehicle optionally carrying an umbilical cable transmitting powerand incorporating a data communication link back to the pipe inlet toavoid power and memory storage being required on board.

6. the vehicle carrying a means of measuring the distance travelledalong the pipe.

In one aspect, therefore, the invention provides apparatus for use inthe characterization of a liquid-filled pipe, which apparatus comprisesa vehicle capable of fitting inside the pipe and of being transported bythe liquid along the pipe, the vehicle carrying rings of severalultrasonic transducers arranged such that during utilisation the ringsare co-axial with the pipe and spaced along the vehicle, and such that

a first ring of transducers operates in the reflection mode purelyradially while

a second and a third ring of transducers operate as a pair displacedaxially to each other, the pair being used in a longitudinal refractionmode in which one ring of the pair emits pulses along the pipe generallyat the critical angle to the wall—that is to say, along directionsspread in a cone so as to accommodate expected variations in thecritical angle of the pipe, liner and debris materials—such that thewave is refracted as waves travelling within the debris, lining and pipewall, and the other ring receives the refracted wave,

the emit ring of transducers of the refraction mode pair also has thecapability of receiving reflected echoes, this being arefraction/reflection mode, and

a fourth ring of transducers is used in a circumferentialrefraction/reflection mode in which each transducer emits pulses in aradial plane but at the critical angle circumferentially to the wallsuch that the wave is refracted as a circumferential wave along thedebris, lining and pipe wall and the ring then receives reflections backalong the same path, the vehicle also carrying means for measuring thedistance travelled along the pipe.

In a second respect the invention provides a Method of characterizing apipe filled with moving liquid, in which Method:

1. the pipe characterizing Apparatus as described above is transportedalong the pipe by the liquid within the pipe.

2. the distance travelled by the Apparatus is measured by one means oranother.

3. ultrasonic pulses are used in radial reflection, in longitudinalrefraction, in longitudinal refraction/reflection and in circumferentialrefraction/reflection modes at regular intervals of distance along thepipe and the received ultrasonic information is stored.

4. the ultrasonic data is processed to determine the materials of thepipewall, the lining of the pipe, the debris accreted on the inside ofthe pipe, the radius of the pipe, the thickness of the pipewall, thelining and the debris, to detect circumferential and longitudinalcracks, damage to the lining and corrosion of the pipewall interior andexterior, and to locate fittings such as joints, valves and junctions.

5. the preferred option is to display processed information as afunction of distance along the pipe.

The preferred form of the vehicle is an autonomous device havingon-board sufficient power and memory for examination of long distancesexceeding one kilometer along the pipe. The vehicle consists of separatemodules small enough to enter the pipe by means of suitable fittings. Inthe water industry existing fittings for entry and removal would be firehydrants from which the whole top assembly can be removed. If nosuitable entry points exist then the vehicle is inserted and recoveredby means of custom-made under-pressure live-launch and recovery stationsthat are fitted onto the pipe when required.

The modules connect together mechanically and the connection includespower and data transmission between modules. The leading module has somemeans such as a collapsable drogue to ensure the flow of liquid providessufficient drag to produce forward propulsion for the string of modules.The modules are designed to be neutrally buoyant to minimize contactwith the pipewall. As well as being neutrally buoyant, the centre ofgravity of each of the modules is below the geometrical centre of themodule. This ensures that on average the module will remain the same wayup and so reduces the requirement for any form of measurement ofvertical alignment. On some modules, such as the ultrasonic modules, itis helpful to have light springs pressing gently against the wall.Centralizing the ultrasonic modules reduces the processing required toextract information from the ultrasonic data.

Ultrasonic measurements are made within the pipe at regular intervalsand the results are presented as a function of distance travelled alongthe pipe. This distance travelled can be measured by means of awall-contacting wheel but the preferred method uses acoustic means tomeasure the distance. The vehicle emits an acoustic pulse at regularintervals which is detected at both ends of the pipe. At launch, thetime for the signal to reach the far end of the pipe, t₀, multiplied bythe speed of sound in the liquid, V_(f), gives the length of the pipeL=v_(f)t₀. During transport, the difference, Δt. between time of arrivalof the pulse at the launch end of the pipe and the time of arrival ofthe pulse at the recovery end of the pipe gives the distance, x, of thevehicle from the launch end as x=½(L+v_(f)Δt). A single endedalternative to this method is to transmit an acoustic signal from oneend of the pipe and have a transponder on the vehicle that emits its ownacoustic reply when it detects the arrival of the first transmittedsignal. At the transmission end of the pipe, the time between emitting asignal and receiving the second one back multiplied by the speed ofsound in the liquid gives the distance to the vehicle. The transmissionand response signals have to be of different character so as todistinguish the transponder response signal from any reflections of theoriginal transmission signal. If an inertial navigation system (INS) isfitted to the vehicle then the distance travelled can be deduced fromthe INS. Even if such a system were available, the preferred methodwould also include acoustic means of measuring distance as backup data.

The combination of ultrasonic transducers is transported axially alongthe inside of a pipe by the vehicle on which they are mounted whilemeasuring in radial reflection mode, in axial refraction mode operatingat an angle to the wall, in axial refraction-reflection mode operatingat an angle to the wall and in circumferential refraction-reflectionmode operating at an angle to the wall such that this complete set maybe used to determine the velocity of ultrasonic waves in the pipewall,the internal diameter of the pipe, the pipe thickness, the velocity ofultrasonic waves in any lining if present, the thickness of any internallining, the presence of corrosion on the internal surface of the pipe,the presence of corrosion on the external surface of the pipe, damage tothe lining, the thickness of any deposits of material on the internalsurface of the pipe or of the lining if present, the presence and extentof circumferential cracks, the presence and extent of longitudinalcracks and the position of features such as bends, offtakes, valves andjoints.

A wide variety of pipes can by examined by the ultrasonic Method,including, among other materials, cast-iron, ductile iron, steelpolyethylene, PVC, and asbestos-cement. The pipes sizes that can beexamined by an autonomous vehicle range from 150 millimeters nominalbore to of the order of one meter in diameter. The pipewall thicknesswill vary according to the liquid, the pressure in the liquid, theflowrate anticipated, the material of the pipewall, the structuralsupport for the pipe and its design lifetime. Typical thicknesses rangefrom a few millimeters to a few centimeters. The linings of pipes dependupon the material of the pipewall and the liquid inside and are notalways used. Typical materials for linings include cement mortar,bituminous layers, epoxy layers and various plastics. Whether or notthere is any accretion to the inside of the pipe, or debris, dependsupon the liquid used and the flowrate involved. In the water industrytypical accretions consist of so-called tubercular growths of hardmaterial initiated by algae at the wall or calcareous deposits. In thepetroleum industry typical accretions tend to be a waxy form ofhydrocarbon. Very often the material of the pipewall, the lining and anyaccretion is not known beforehand. In this invention it is refractionmode measurements, in identifying the various materials by determiningthe speed of sound, which allow reflection mode measurements todetermine radii and thicknesses of materials. The further combination ofall modes of measurement allows for identification of corrosion,longitudinal cracks, circumferential cracks and fittings such as valves,joints and junctions.

The number of ultrasonic transducers used determines the circumferentialresolution of the measurements. For instance, in reflection mode a ringof eight transducers used inside a 150 mm bore pipe gives acircumferential resolution of 59 mm. The same circumferential resolutionapplies to a pair of rings of transducers used in refraction mode, andto a ring used in longitudinal refraction/reflection mode and to a ringused in circumferential refraction/reflection mode. The inventiondescribes the Method of carrying out the above measurements, namely withultrasonics in reflection, refraction and refraction-reflection modes,while transporting the set of measurement transducers axially along theinside of the pipe. The speed of ultrasonic waves in the solid andliquid is high. For example typical speeds of sound in iron and in waterare 5000 m/s and 1500 m/s respectively. These are sufficiently highcompared with the speed with which the transducers can be translatedaxially along a pipe by an autonomous vehicle, typically of order 1 m/sor less, that measurements may be taken to vary continuously withdistance along the pipe. All the measurements are taken on a repeatedbasis as the transducers are transported along the pipe. The rate ofrepetition of the measurements compared with the rate of translationalong the pipe determines the axial resolution of the measurements.

For example, if the axial translation rate of the module bearing theultrasonic transducers was 0.1 m/s then a repetition rate of ten timesper second gives a longitudinal resolution of 10 mm. Ultrasonictransducers for both emit and receive are of piezo-electric type such asmarketed by Morgan Matroc Limited of Transducer Products Division,Thornhill, Southampton, Hampshire S019 7TG. There are typically eight ormore equally disposed in a ring around the body of the transportingmodule in any one transverse plane. One such ring is used for reflectionmode measurements. A pair of rings axially displaced relatively to eachother are used for refraction and refraction-reflection modemeasurements along the pipe. A further ring is used for refractionmeasurements circumferentially around the pipe.

Specific embodiments of the invention are now be described by way ofexample with reference to the accompanying drawings, 1, 2, 2 a, 3, 4, 5and 6 in which:

1. FIG. 1 is an explanatory sketch concerning Snell's law of refraction.

2. FIG. 2 shows a schematic view of a pipe with a cut-away sectionshowing a vehicle inside with four rings of ultrasonic transducersaround the periphery.

FIG. 2a is a simplified schematic sectional view of a pipe illustratinga multiple-module vehicle inside supported by spacer means and utilizinga collapsible drogue.

3. FIG. 3 shows a longitudinal (along its length) cross-section througha pipe with an ultrasonic transducer emitting and receiving wavesradially.

4. FIG. 4 shows a transverse cross-section through the pipe with anultrasonic transducer emitting and receiving waves radially.

5. FIG. 5 shows a longitudinal (along its length) cross-section througha pipe, with one ultrasonic transducer emitting waves at an angle to thelongitudinal axis of the pipe and a second transducer receiving therefracted waves at a position displaced axially from the emitter.

6. FIG. 6 shows a transverse cross-section through a pipe with anultrasonic transducer emitting waves at an angle to the pipe in thecircumferential plane. The waves are being reflected by a defect in thepipe and then received by the same transducer as emits the pulse.

FIG. 2 shows a schematic view of a pipe 32 filled with liquid 2 with acut-away section 31 showing a module 33 inside with four rings ofultrasonic transducers around the periphery. One pair of rings consistof a ring 35 for transmission and ring 38 for reception of longitudinalrefraction mode waves. The transmission ring 35 also serves to receivelongitudinal refraction/reflection mode waves. Ring 36 is used forradial reflection mode waves. Ring 37 is used for circumferentialrefraction/reflection mode waves. The vehicle serves to transport thearrangement of transducers along the pipe. The module 33, which may formpart of a chain of such modules 33, 33 a (FIG. 2a), is free floating Inthe pipe. It is centralized by spacer means 39, preferably in the formof springs which press gently against the pipe wall. The modules aredrawn along by the drag of the flow of liquid 2 upon a collapsibledrogue 40. The other modules 33 a have various functions such as powersupply and memory storage. The modules are small enough to enter thepipe by means of suitable fittings. The cross-section through the pipe32 shows the pipe wall 3 and internal lining 4. The debris on the liningis not shown. FIG. 3 shows an axial cross-section through a pipe withcenterline 1 containing liquid 2 and showing on one side of centerline 1the pipe wall 3, lining 4 to the pipe wall 3, and deposit 5 on thelining 4. An ultrasonic transducer 6 mounted on the module 33 is shownemitting a pulse which is transmitted 7 through the liquid and reflectsat the interface 8 with the deposit 5, at the interface 9 with thelining 4, at the front face 10 of the pipe wall 3, and at the back face11 of the pipe wall 3. The width of the pulse need not extend greatlybeyond the size of the transducer 6. (The width is shown delimited bythe parallel lines 27.) These reflections travel back along the sameroute to the transducer 6 at which they are detected. Within any ofthese materials there may be multiple reflections.

The timing of return of the reflected pulses indicates the thickness ofthe deposit 5, of the lining 4, of the pipewall 3 and the distance ofthe transducer 6 from the wall. The transducer is at a known distancefrom the centreline of the pipe 1, and so the radius of the pipe isdetermined as long as the speed of sound in the solid materials isknown. The liquid material must have a known consistent speed of sound.

A defect 18 is shown on the outside of the pipewall 3, and a seconddefect 19 is shown on the inside of the pipewall 3. The timing of thereturn reflections from such defects gives a measure of the depth of thedefects. The same applies within the lining.

In FIG. 3 the transmitted wave 7 is shown without any lateral spread 27because there is no need to include significant lateral spread beyondthe width of the emitting transducer 6, although there will inevitablybe some spread by diffraction.

In FIG. 4 is shown a radial cross-section through the same arrangementof reflection mode transducers 6. Each of eight transducers 6 is shownlying on a circle about centreline 1. The transmitted wave 7 is shown toextend over an arc delimited by the dashed lines 24 determined by thesize of the transducer 6. The resulting measurements based onreflections are an average echo received from the surface area delimitedby the arc 24 and this arc determines the angular resolution. In FIG. 5is shown an axial cross-section through the pipewall 3 with anultrasonic transducer 12 transmitting a wave 14 at an angle to the wall,shown with a deposit 5 on a lining 4. The wave 14 is refracted into thevarious materials at the critical angle 16, travels along the materials,and refracts out again at the same critical angle 17, and the wave 15 isreceived at a second transducer 13 displaced axially from the first one12 from which the wave was emitted. The critical angle 16 depends uponthe material, and so the waves 14 are deliberately spread in a coneshown by dashed lines 25 to accommodate variations expected in angle 16.A circumferential defect 18 is shown, and this could represent a crack,corrosion, or a fitting such as a flange, valve or joint to aside-branch pipe. The travel time through the different materialsindicates the speed of sound in those materials. The speed of sound inthe materials identifies the materials of the pipewall, the lining andthe debris. In addition, the speed of sound in each material is neededto multiply the transit times of reflection mode radial pulses so as toconvert these times into absolute distances including thickness ofdebris, thickness of lining and thickness of pipewall. The transmittedwave 14 is reflected at a defect 18 when the defect lies between thetransducers 12 and 13, and the reflected wave is received at the sametransducer 12 as emitted the original pulse. Reflections from the defect18 indicate the presence of such defects.

FIG. 6 shows a radial cross-section through the pipe with a transducer20 emitting a wave 21 at the critical angle to the wall such that thewave travels circumferentially around the pipewall 3 within the wallmaterial. A longitudinal defect reflects the wave back to the emittingtransducer 20, and the timing of the return indicate the position of thedefect. To accommodate variations in critical angle, the wave 21 extendsacross a cone delimited in FIG. 6 by a dashed line 26.

In reflection mode, a pulse is emitted by the ultrasonic transducer in aradial direction, normal to the pipe wall, and the reflections fromsurfaces normal to the path of the ultrasonic wave are received,commonly at the same ultrasonic transducer. Surfaces normal to the pathof this reflection mode pulse could be the inner surface of any depositand the interfaces between deposits, lining, corrosion and the pipewallitself at inner and outer surfaces.

The timing of radial reflection mode signals gives an indication of theradial dimensions of the pipe, thickness of the pipewall, pipe liningand deposits. Suppose, for example, that the speed of sound in theliquid is v_(f) and the first pulse reflection is received after t_(f)seconds. Then the distance travelled through the liquid from thetransducer to the innermost face of the wall, possibly the debris, andback to the transducer is the product 0.5v_(f)t_(f). The next reflectionis from the interface between the debris and the lining. If thisreflection is received at time t_(d) and the speed of sound in thedebris was v_(d) then the thickness of the debris is the product0.5(t_(d)−t_(f)) v_(d). The same principle is applied to any number ofreflections provided that it is possible to distinguish them. Anyanomalies in timing indicate the presence of various defects. Forexample, if measurements continuously yield the thickness of thepipewall as the transducers travel axially along the pipe and then atsome point there is a much reduced time of arrival from the outside faceof the pipewall, this indicates that there is substantial corrosion atthat point on the wall. There may be multiple reflections within any ofthe materials in the path and most likely within the pipewall. Suchmultiple reflections result in repeated echoes at the receiver and serveto reinforce the calculation of material thickness. The time betweenthese multiple echoes multiplied by the speed of sound in the materialyields repeated estimates of the thickness of the material.

Timing information gives the radial distance from the transducer to thefirst reflecting face, such as the debris, since the speed of sound inthe liquid is known. However the timing information cannot give thedimensions of the materials, debris, lining or pipewall, unless thespeed of sound in these material is known accurately. The speed of soundin the materials comes from timing of the pulses transmitted by therefraction mode pulse, described next.

In refraction mode, a pulse is emitted by the ultrasonic transducer atan angle to the pipe wall known as the angle of incidence. The angleinvolved is determined by Snell's law which matches the speed of soundin two media by the sine of the angles subtended to the normal at theinterface by the waves in the two materials. In FIG. 1 for example, awave 7 is shown being transmitted from liquid 2 into debris 5 at anangle of incidence 30 to the radial normal 34, from debris into thelining at an angle 29, from lining into pipewall at an angle 28. If thespeed of sound in liquid, debris, lining and pipewall is represented byV_(f), V_(d), v_(l), and v_(p) respectively and the angles 30, 29, 28are represented by α_(f), α_(d), and α_(l), respectively then${{\sin \quad \alpha_{f}} = \frac{v_{f}}{v_{p}}},\quad {{\sin \quad \alpha_{d}} = \frac{v_{d}}{v_{p}}},\quad {{{and}\quad \sin \quad \alpha_{l}} = {\frac{v_{l}}{v_{p}}.}}$

The wave is refracted into the pipe wall and travels along the wall. Theangle of incidence of the wave with the pipe that results in a wavewithin the pipewall travelling parallel to the pipewall is referred toas the critical angle for refraction into the pipewall. The criticalangle for refraction into the lining will be slightly greater and thecritical angle for refraction into the debris will be slightly greaterstill. By transmitting waves over a small range of angles, all thecritical angles are included. At angles of incidence greater than thecritical angle the waves totally reflect at the interface. At angles ofincidence less than critical the introduction of longitudinal waves intothe pipewall is less efficient. The plane containing the normal to thepipewall and the projected wave can be longitudinal, in which case thedirection of travel through the solid is longitudinal (axial) down thepipe. Alternatively, the plane containing the normal to the pipewall andthe projected wave can be transverse to the pipe in which case thedirection of travel in the solid is circumferential. As the pulsetravels along it is continuously refracting out again from the wall,back into the liquid region, and some of these refracted transmissionwaves are detected at a second, receive, ultrasonic transducer. Therefracted waves can also meet areas of reflection along the pipewall,such as cracks, corrosion and joints, and the reflected waves aredetected at the same transducer from which the wave was emitted.

The first arrival at the receive transducer in the refraction mode isthat of waves transmitted through the pipewall since that material hasthe highest speed of sound, around 500 m/s. The length of the paths inthe refraction mode are determined by the distance apart of the emit andreceive transducers and this is prescribed a-priori by the geometry.Since the speed of sound in the liquid can be presumed to be known, thetime of the first arrival pulse yields the speed of sound in thepipewall. Suppose the distance from the transducer to the debris isx_(f), the thickness of debris is X_(d), thickness of lining is x_(l),the distance between emit and receive transducers is L and the time toreceive the pulse is t then we have for the speed of sound in thepipewall the expression$v_{p} = {\frac{1}{t}{\left( {L - \frac{2x_{f}}{\cos \quad \alpha_{f}} - \frac{2x_{d}}{\cos \quad \alpha_{d}} - \frac{2x_{l}}{\cos \quad \alpha_{l}}} \right).}}$

We notice that x_(d), x_(l) and the angles α_(f), α_(d) and α_(l) areunknown until the speeds of sound in the materials are determined. Thismeans the various equations are weakly coupled. The coupling is weakbecause the distances x_(d) and x_(l) are small in comparison with theradial distance X_(f). The equations quickly converge if solved by aniterative scheme whereby initially the small unknowns are ignored andthen increasingly refined at each iteration.

Similarly, subsequent arrivals through the lining and debris yield thespeed of sound in those materials. Partial reflection of longitudinalrefraction mode signals indicates the presence of a defect such as acircumferential crack whereas total reflection of the longitudinalrefraction mode signals indicates the presence of a fitting such as aflange. Partial reflection of circumferential refraction mode signalsindicates the presence of a defect such as a longitudinal crack. As thetransducers are transported axially along the pipe, the reflection modetransducers receive a temporarily distorted signal as they pass afitting such as a flange. Consequently both the reflection andrefraction mode signals can detect corrosion, cracks and fittings.Comparison of events detected by the reflection mode signals andrefraction mode signals serves to distinguish cracks from corrosion andvarious types of fitting, especially as the repeated use of thetransducers allows a learning process to be carried out whereby thequalitative shape of reflections is used to distinguish betweendifferent features such as cracks, corrosion and fittings.

What is claimed is:
 1. Apparatus for use in the characterization of aliquid-filled pipe, which apparatus comprises a vehicle capable offitting inside the pipe and of being transported by the liquid along thepipe, the vehicle carrying rings of several ultrasonic transducersarranged such that during utilization the rings are co-axial with thepipe and spaced along the vehicle, and such that a first ring oftransducers operates in the reflection mode purely radially while asecond and a third ring of transducers operate as a pair displacedaxially to each other, the pair being used in a longitudinal refractionmode in which one ring of the pair emits pulses along the pipe generallyat the critical angle to the wall—that is to say, along directionsspread in a cone so as to accommodate expected variations in thecritical angle of the pipe, liner and debris materials—such that thewave is refracted as waves traveling within the debris, lining and pipewall, and the other ring receives the refracted wave, the emit ring oftransducers of the refraction mode pair also has the capability ofreceiving reflected echoes, this being a refraction/reflection mode, anda fourth ring of transducers is used in a circumferentialrefraction/reflection mode in which each transducer emits pulses in aradial plane but at the critical angle circumferentially to the wallsuch that the wave is refracted as a circumferential wave along thedebris, lining and pipe wall and the ring then receives reflections backalong the same path, the vehicle also carrying means for measuring thedistance traveled along the pipe, and a collapsible drogue to ensurethat in use the flow of liquid provides sufficient drag to produceforward propulsion of the vehicle.
 2. Apparatus as claimed in claim 1,wherein there are eight or more transducers equally disposed in eachring.
 3. Apparatus as claimed claim 1, wherein the vehicle carries datastorage means in which to save the ultrasonic information which inoperation is captured at regular intervals of distance along the pipe,together with power supply means for the transducers and associated datagathering and data storage.
 4. Apparatus as claimed claim 1, wherein thevehicle consists of a plurality of separate connected modules smallenough to enter the pipe by means of suitable fittings.
 5. Apparatus asclaimed in claim 1, wherein the vehicle is neutrally buoyant, and hasits center of gravity below the vehicle's geometric center.
 6. Apparatusas claimed claim 1, wherein the vehicle has spacer means which in usecentralize the vehicle within the pipe.
 7. Apparatus as claimed in claim6, wherein the spacer means is a plurality of springs extendinglaterally from the vehicle, which springs in use press gently againstthe pipe wall so as to centralize the vehicle.
 8. Apparatus as claimedclaim 1, wherein the vehicle's distance measuring means are acousticmeans, which in use involve measuring the time of travel of acousticpulses along the pipe.
 9. Apparatus for use in the characterization of aliquid-filled pipe, which apparatus comprises a vehicle capable offitting inside the pipe and of being transported by the liquid along thepipe, the vehicle carrying rings of several ultrasonic transducersarranged such that during utilization the rings are co-axial with thepipe and spaced along the vehicle, and such that a first ring oftransducers operates in the reflection mode purely radially while asecond and a third ring of transducers operate as a pair displacedaxially to each other, the pair being used in a longitudinal refractionmode in which one ring of the pair emits pulses along the pipe generallyat the critical angle to the wall—that is to say, along directionsspread in a cone so as to accommodate expected variations in thecritical angle of the pipe, liner and debris materials—such that thewave is refracted as waves traveling within the debris, lining and pipewall, and the other ring receives the refracted wave, the emit ring oftransducers of the refraction mode pair also has the capability ofreceiving reflected echoes, this being a refraction/reflection mode, anda fourth ring of transducers is used in a circumferentialrefraction/reflection mode in which each transducer emits pulses in aradial plane but at the critical angle circumferentially to the wallsuch that the wave is refracted as a circumferential wave along thedebris, lining and pipe wall and the ring then receives reflections backalong the same path, the vehicle also carrying means for measuring thedistance traveled along the pipe, and the vehicle is neutrally buoyant,and has its center of gravity below the vehicle's geometric center. 10.Apparatus as claimed in claim 9, wherein there are eight or moretransducers equally disposed in each ring.
 11. Apparatus as claimed inclaim 9, wherein the vehicle carries date storage means in which to savethe ultrasonic information which in operation is captured at regularintervals of distance along the pipe, together with power supply meansfor the transducers and associated date gathering and data storage. 12.Apparatus as claimed in claim 9, wherein the vehicle consists of aplurality of separate connected modules small enough to enter the pipeby means of suitable fittings.
 13. Apparatus as claimed in claim 9,wherein the vehicle has spacer means which in use centralize the vehiclewithin the pipe.
 14. Apparatus as claimed in claim 13, wherein thespacer means is a plurality of springs extending laterally from thevehicle, which springs in use press gently against the pipe wall so asto the pipe wall so as to centralize the vehicle.
 15. Apparatus asclaimed in claim 9, wherein the vehicle's distance measuring means areacoustic means, which in use involve measuring the time of travel ofacoustic pulses along the pipe.
 16. Apparatus for use in thecharacterization of a liquid-filled pipe, which apparatus comprises avehicle capable of fitting inside the pipe and of being transported bythe liquid along the pipe, the vehicle carrying rings of severalultrasonic transducers arranged such that during utilization the ringsare co-axial with the pipe and spaced along the vehicle, and such that afirst ring of transducers operates in the reflection mode purelyradially while a second and a third ring of transducers operate as apair displaced axially to each other, the pair being used in alongitudinal refraction mode in which one ring of the pair emits pulsesalong the pipe generally at the critical angle to the wall—that is tosay, along directions spread in a cone so as to accommodate expectedvariations in the critical angle of the pipe, liner and debrismaterials—such that the wave is refracted as waves traveling within thedebris, lining and pipe wall, and the other ring receives the refractedwave, the emit ring of transducers of the refraction mode pair also hasthe capability of receiving reflected echoes, this being arefraction/reflection mode, and a fourth ring of transducers is used ina circumferential refraction/reflection mode in which each transduceremits pulses in a radial plane but at the critical anglecircumferentially to the wall such that the wave is refracted as acircumferential wave along the debris, lining and pipe wall and the ringthen receives reflections back along the same path, the vehicle alsocarrying means for measuring the distance traveled along the pipe, thedistance measuring means being acoustic means, which is use involvemeasuring the time of travel of acoustic pulses along the pipe. 17.Apparatus as claimed in claim 16, wherein there are eight or moretransducers equally disposed in each ring.
 18. Apparatus as claimed inclaim 16, wherein the vehicle carries date storage means in which tosave the ultrasonic information which in operation is captured atregular intervals of distance along the pipe, together with power supplymeans for the transducers and associated date gathering and datastorage.
 19. Apparatus as claimed in claim 16, wherein the vehicleconsists of a plurality of separate connected modules small enough toenter the pipe by means of suitable fittings.
 20. Apparatus as claimedin claim 16, wherein the vehicle has spacer means which in usecentralize the vehicle within the pipe.
 21. Apparatus as claimed inclaim 16, wherein the spacer means is a plurality of springs extendinglaterally from the vehicle, which springs in use press gently againstthe pipe wall so as to centralize the vehicle.